The present invention is directed to a method of and system for computed tomography (CT) density image reconstruction. More particularly, the present invention is directed to the three-dimensional reconstruction from two-dimensional projections acquired with x-ray cone-beam CT and single photon emission computed tomography (SPECT) scanners. Even more particularly, the present invention is directed to a method of and system for intravenous volume tomographic digital angiography imaging.
For about the past twenty years, computerized tomography has revolutionized diagnostic imaging systems as well as non-destructive test imaging techniques. Conventional CT scanners use a fan-shaped x-ray beam and one-dimensional detector in order to reconstruct a single slice with a single scan of an object. However, current CT technology is limited by a trade-off between high longitudinal resolution and fast volume scanning. One method which has been utilized to address the shortcomings of CT scanner technology is the use of cone-beam tomography. A cone-beam volume CT scanner uses a cone-beam x-ray source and a two-dimensional detector to reconstruct the whole volume of an object with a single scan of that object. The data obtained from the scan of the object is processed in order to construct an image that presents a two-dimensional slice taken through the object. The current technique for reconstructing an image from 2-D is referred to in the art as the filtered back projection technique. That process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units" which are then used to control the brightness of a corresponding pixel on a cathode ray display.
In a 3-D scan technique, a cone-shaped x-ray beam is used which diverges to form a cone-beam that passes through the object and impinges on a two-dimensional array of detector elements. In that manner, the volume scanning time of a 3-D object can be at least 10 times shorter than a standard CT on a spiral CT. In contrast to existing CT with an intraslice plane resolution of 1.0 lp/mm, the reconstructions of cone beam CT will have isotropic spatial resolution along all three axes (0.5-2.0 lp/mm). Each view is thus a 2-D array of x-ray attenuation measurements and the complete scan produces a 3-D array of attenuation measurements.
At present, either of two methods are commonly used to reconstruct a set of images from the acquired 2-D attenuation measurements. The first technique is that developed by Feldkamp, Davis & Kress, which is described in "Practical Cone-Beam Algorithm", J. Opt. Soc. Am., Vol. I, pp. 612-619 (1984). The Feldkamp, et al. technique, which uses an algorithm which was derived using approximations of a tilted fan beam formula, is a convolution-back projection method which operates directly on the line integrals of the actual attenuation measurements. That method can be easily implemented with currently available hardware and is a good reconstruction for images at the center or "mid-plane" of the cone-beam. While the algorithm of Feldkamp, et al. provides excellent computational efficiency and minimal mechanical complexity in data acquisition, its major shortcoming is that it is based on single circle cone-beam geometry. Single circle cone-beam geometry, in which the source always lies on a circle, cannot provide a complete set of data to exactly reconstruct the object. For that reason, Feldkamp, et al.'s algorithm causes some unavoidable distortion in the non-central transverse planes, as well as resolution degradation in the longitudinal direction.
In order to address the problems of Feldkamp's algorithm, several other approaches have been proposed using different cone-beam geometries including dual orthogonal circles, helical orbit, orthogonal circle-and-line, and Smith's curve. Such geometries can achieve exact reconstructions when using the approach of Tuy, Smith, or Gangreat.
In addition to the Feldkamp, et al. approach for analytic cone-beam reconstruction, a second commonly used method is that disclosed by Pierre Grangeat in, "Mathematical Framework of Cone-Beam 3-D Reconstruction Via the First Derivative of the Radon Transform", Mathematical Methods in Tomography, Herman, Lewis, Natterer (eds.) Lecture Notes in Mathematics, No. 1497, pp. 66-97, Spring Verlag (1991). That algorithm provides an accurate solution to the image reconstruction task based on a fundamental relationship between the derivative of the cone-beam plane integral through the derivative of the parallel beam plane integral. While the Grangeat method is theoretically accurate, it requires mathematical operations that can be solved only using finite numerical calculations that are approximations. Thus, errors can be introduced by the implementation of the Gangreat method that can be greater than those produced using the Feldkamp, et al. method and such errors are not correlated with the cone-beam angle.
A third method has been disclosed by H. K. Tuy in "An Inversion Formula for a Cone-Beam Reconstruction", SAIM J. Appl. Math. 43, pp. 546-552 (1983). Using Tuy's approach, in order to generate a complete or sufficient set of data, every plane which passes through the imaging field of view must also cut through the orbit of the focal point at least once. The single plane or orbit of Feldkamp, et al. does not satisfy this condition.
Still yet another approach that has been proposed is the inversion of the cone-beam data sets if the assumption is made that for any line that contains a vertex point and a reconstruction point, there is an integer M which remains constant for the line such that almost every plane that contains this line intersects the geometry exactly M times. Mathematical improvement to the reconstruction algorithms was described in an article by B. D. Smith entitled "Cone-Beam Tomography: Recent Advances and a Tutorial Review," Opt. Eng., Vol. 29 (5) pp. 524-534 (1990). However, such an integer requirement condition is too restrictive for practical application since the only known source point geometry which meets that condition is a straight line.
Two somewhat recent patents were issued in the United States directed to the cone-beam reconstruction problem. The first, U.S. Pat. No. 5,170,439 to Zeng, et al., was issued on Dec. 8, 1992 and utilizes the above-described cone-beam reconstruction method using combined circle and line orbits. However, that technique requires the removal of redundant and unnecessary data, which necessarily requires more computing time and complexity than the method and system of the present invention.
Another approach to the reconstruction of images from cone-beam data is disclosed in U.S. Pat. No. 5,400,255, which issued to Hu on Mar. 21, 1995. The methodology disclosed in the Hu patent represents a minimal improvement from Feldkamp's algorithm and it is still an approximate method based on a single circle cone beam geometry. It cannot result in exact reconstruction and it is not acceptable in many clinical applications when the cone angle is large.
In contrast to the prior art approaches, the present invention discloses an exact cone-beam reconstruction system and method using a circle-plus-arc data acquisition geometry in which the locus of a source and a detector is a circle plus an orthogonal arc. In that manner, the best image quality of a cone-beam volume CT is achieved without introducing any additional mechanical complexity compared to a regular CT gantry. If the locus of an x-ray source and a detector is a single circle during cone-beam scanning (single circle cone-beam geometry), an incomplete set of projection data will be acquired. The incompleteness of the projection data results in some unavoidable blurring in the planes away from the central z plane and a resolution loss in the z direction (i.e., Feldkamp, et al.'s algorithm). The reconstruction error due to the incompleteness of the projection data could be up to 50% of the signal when using Feldkamp, et al.'s algorithm with a 22.degree. cone angle. However, using the data acquisition geometry of the present invention, the locus of an x-ray source and a detector is a circle plus an arc perpendicular to the circle. That corresponds to rotating the x-ray tube and detector on the gantry, and then acquiring the arc projections on a perpendicular arc while tilting the gantry at a relatively small angle (.+-.15.degree. to .+-.30.degree.). Such geometry results in a complete set of data for an object with a 25-40 cm length in the z direction, which corresponds to a 37-60 cm field size at the detector in the z direction with a magnification of 1.5. Using the system and method of the present invention, the 3-D reconstruction is exact and no image blurring or resolution loss occurs.
The method and system of the present invention is based upon the three-dimensional Radon transform. The algorithm used with the present invention first transforms the cone-beam projections acquired from a circle-arc orbit into the first derivative of the 3-D Radon transform of an object using Grangeat's formula. Then, the object function is reconstructed using the inverse Radon transform. In order to reduce the interpolation errors in the re-binning process required by Grangeat's formula, new re-binning equations have been derived exactly, therefore transforming 3-D interpolations into one-dimensional interpolations. The inventive cone-beam acquisition method and system disclosed herein provides a complete set of projection data such that the cone-beam image reconstruction algorithm achieves exact reconstructions. The result is a 3-D cone-beam reconstruction which introduces no obvious artifacts and only a practical acceptable reduction of reconstruction accuracy.
The 3-D volume tomographic imaging and system described above can also be used to achieve a 3-D or volume tomographic digital angiography imaging method and system which is capable of providing clinically useful 3-D vascular images for enhancing diagnostic and therapeutic decisions. In particular, the volume tomographic digital angiography imaging method and system disclosed herein is particularly useful for intravenous (IV) volume tomographic digital angiography (IV-VTDA). Such IV-VTDA is superior to conventional angiography because it provides a direct, unambiguous and accurate 3-D measurement of stenosis and other irregularities and malfunctions, including the caliber, geometry and spatial orientation in the structure. Moreover, the IV-VTDA method and system disclosed herein requires only a single IV injection of contrast media and uses fast volume scanning, thus reducing the invasiveness of the procedure as well as the procedure time, while also providing a substantial reduction in the total x-ray exposure to the patient.
A need for the accurate and detailed assessment of atherosclerotic disease has been reemphasized by the growth of new therapeutic techniques, such as thrombolysis, endarterectomy, atherectomy, angioplasty, embolization and the placement of vascular stents, as well as the need to facilitate and improve the success rate of such therapeutic procedures. Cerebrovascular disease is the third leading cause of death in the United States and claims approximately 500,000 new victims each year. New surgical and endovascular techniques greatly improve patient survival and their quality of life. Thus, there has been an increase in the therapeutic procedures enumerated above in the last several years. As the result of such procedures, patient survival has increased and the quality of the patient's life has improved.
The identification of patients who can benefit from a specific therapeutic procedure requires both accurate and detailed information about the severity of the stenosis, their geometry and their spatial orientation. However, most therapeutic decisions are based on information obtained through standard projectional angiographic techniques. Projection images using such standard projectional techniques do not provide sufficient information with which to detect and completely characterize all vascular lesions. That lack of complete data impairs the ability of the physician to determine the optimal therapeutic procedure. Obviously, an inappropriate choice of intervention based on improper knowledge of the patient's anatomy can lead to unnecessary interventions, a sub-optimal outcome, injury or death.
As discussed above, all of the standard projectional angiographic techniques contain major shortcomings with respect to providing a complete characterization of vascular lesions. For example, intraarterial (IA) digital subtraction angiography (IA-DSA), which is currently used for examining most patients for vascular disease, has two principle limitations. First, IA-DSA provides only a 2-D projection of 3-D anatomical structures. Second, IA-DSA images are of reduced usefulness due to vessel overlap, particularly when non-selective injections are used. Obviously, the knowledge of the geometry of the stenosis and the spatial orientation of the arteries is a major step in the performance of successful surgical or transvascular interventional procedures. Thus, using IA-DSA techniques, multiple views are utilized to attempt to detect all lesions as well as to evaluate the geometry of the stenosis and to integrate the 2-D views into correct spatial relationships. That, in turn, requires the use of multiple contrast injections as well as a multiple series of x-ray exposures.
However, even with multiple views, the number of views is limited, which often results in non-detected lesions because of the failure to achieve orthogonal projection and overlap. Consequently, the angiographic procedure can become prolonged, increasing patient morbidity from lengthened catherterization time, increasing contrast as well as the radiation dose, while also increasing procedure costs. There is also an added risk of complications related to percutaneous cannulation of an artery and the manipulation of the IA catheters and wires in critical vessels which are often affected by vascular disease. The risk of procedure related vascular injury and stroke is also present, with major morbidity. Moreover, such angiography technique is frequently repeated as the vascular disease progresses, thus multiplying costs and risks.
In the past, in order to avoid the shortcomings and risks of IA-DSA, attempts have been made to utilize intravenous digital subtraction angiography (IV-DSA). However, in addition to the shortcomings inherent with all types of DSA, there are two additional important technical deficiencies which are specific to IV-DSA. First, image misregistration often occurs due to patient motion. Such misregistration too often masks the vascular anatomy to be imaged. Second, there is an inability to attain a sufficiently high concentration of contrast media through intravenous injection to overcome the quantum noise inherent in the DSA technique. Due to those deficiencies, the resulting image is generally of poor quality and, thus, IV-DSA has become an infrequently used clinical technique.
In the past fifteen years, many attempts have been made to improve the image quality of IV-DSA. Such techniques have been only partially successful in reducing the severity of motion artifacts and in improving problems with vessel overlaps. Thus, even with such improved IV-DSA techniques, there still exists a significant amount of missing 3-D information which would be very useful to obtain.
An improvement over IA-DSA can be obtained by incorporating the volume tomographic imaging principles discussed herein with digital angiography. As is disclosed in more detail herein, a cone-beam volume CT scanner using an image intensifier coupled to a CCD camera as a 2-D detector can be used to obtain CT-like 3-D reconstructions of blood vessels from a single IA contrast media injection and a single fast volume scan. In contrast to the DSA technique, such an image intensifier-based volume based tomographic imaging method and system provides the ability to tomographically isolate an object of interest, such as a blood vessel, from the structures in an adjacent plane, such as other blood vessels or bone. The 3-D reconstructions eliminate vessel overlap and provide a complete, true 3-D description of the vascular anatomy. Such reconstructions have isotropic spatial resolution along all three axes. Others have reported similar results on selective intra-arterial volume tomographic angiography reconstructions, thus demonstrating the advantages of IA-VTDA over IA-DSA. See, for example, an article entitled "3 D computed x-ray angiography: first in vivo results," by D. St-Felix, R. Campagnalo and Y. Rolland, et al. in Radiology 1992, 185:304, a paper presented by R. Fahrig, A. J. Fox and D. W. Haldsworth, entitled "Three-Dimensional CT Angiography from a C Arm Mounted XRII," which was presented at RSNA 82nd Scientific Assembly, Dec. 1, 1996, and a paper presented by K. Sekihara, H. Kawai, K. Yamamoto and T. Kumazaki, entitled "Cone Beam CT Angiography," at Proc. of JAMIT Frontier'95, pp. 23-28, 1995.
One of the drawbacks of IA-VTDA is that it is based upon IA injections, which are generally much more invasive than IV injections. Although when compared to DSA, IA-VTDA represents a significant advance, IV-VTDA represents an even greater advance compared to IA-DSA because it has all of the advantages IA-VTDA has over IA-DSA and at the same time makes the angiographic procedure much safer. IV-VTDA also provides a significant reduction in the cost of the angiographic procedure because it eliminates the need for arterial puncture and catherterization.
One of the difficulties of using IV-VTDA in place of IA-VTDA is that IV injections result in a much lower signal compared to IA injections. Whereas a selective IA injection results in almost no dilution of injected iodine concentration and a non-selective IA injection results in a factor of 3-4 dilution, the dilution of central and peripheral IV injections depends on cardiac output, transit time, venous capacitance, the injection rate and the length of the injection. Dilution factors on the order of 20:1-30:1 are common. The result is that an IV-VTDA system must compensate for a significantly lower signal compared to IA-VTDA, which in turn requires that the IV-VTDA system have a much better low contrast resolution than an IA-VTDA system.
There are two IV injection protocols. One is the central IV injection which is performed at the vena cava near the right atrium. The other is the peripheral IV injection, which is performed through the antecubital fossa or other peripheral veins. If necessary, veins in both antecubital fossae can be injected simultaneously to achieve even higher rates of contrast administration intravenously. The injection can be performed using an injector, and contrast solution can be iodinated contrast materials.
Other modalities could potentially also be used for 3-D angiographic imaging, such as helical CT, magnetic resonance angiography (MRA) and ultrasound (US). However, IV-VTDA is clearly preferable to all three of these modalities.
Spiral CT angiography (CTA), while having proven useful for the evaluation of cerebrovascular and aorto iliac disease, has some major disadvantages when compared to IV-VTDA. First, the long volume scan time of CTA limits the rate of contrast injection and at least a 30 second breathhold is required by the patient. Therefore, CTA is more sensitive to patient motion than IV-VTDA techniques. Also, due to tube loading limitations, the resolution in the section direction of CTA is practically limited, and small-vessel resolution may be limited by partial volume effects. IV-VTDA, on the other hand, requires a much shorter volume scanning time, which allows a higher contrast media injection rate so that a much higher IV injected iodine signal can be achieved, which produces a better image quality, requires less contrast media and a much smaller tube loading. Thus, compared to CTA techniques, IV-VTDA can cover a much larger segment of the body in the direction orthogonal to the slices than conventional CTA, with a single injection.
It is contemplated that the use of IV-VTDA for imaging a patient's body will produce significant benefits over the images produced by the present CTA techniques. IV-VTDA is also superior for cross-sectional pulmonary angiography because of its shorter breathhold requirements as well as the isotropic resolution it attains. Furthermore, IV-VTDA can also be used for lower extremity angiography where spiral CTA cannot be used because of the limited tube capacity and total amount of contrast media which can be safely administered to a patient.
MRA has already proven useful for the evaluation of vascular disease. However, current MRA procedures have some deficiencies, including limited spatial resolution, overestimation of stenosis and other artifacts, particularly at regions of flow disturbances, tradeoffs between maximizing the field of view (FOV), signal to noise ratio and spatial resolution and relatively long scanning times, which make it sensitive to patient motion. While many attempts have been made to solve these problems, even if they are finally solved, the IV-VTDA system and method of the present invention will be less expensive. IV-VTDA can also be used for patients with contraindications to MR scanning, such as claustrophobia, pacemakers, cerebral aneurysm clips, implanted defibrillators, previous surgery with metal implants, or prior trauma with residual metal fragments, and will also allow visualization inside metallic endovascular stents.
Transcutaneous duplex ultrasound (US) has the advantages of real-time non-invasive imaging that provides spectral information in a relatively inexpensive package while also providing extraluminal information. However, compared to IV-VTDA, a principle limitation of ultrasound is the need for an appropriate acoustic window. Intervening air or bone prevent the acquisition of diagnostic information at a substantial number of potential vascular sites. For that reason, ultrasound has a primary diagnostic role in the carotid and lower extremity arteries, but is not useful in adult (closed fontanelle) skull and central chest areas and is limited in use in the deep abdomen. Furthermore, calcific plaque obscures visualization, often right at the stenosis. Other disadvantages of ultrasound include a limited FOV, dependence on Doppler angle, dependence on operator skill, and the inability to distinguish total occlusion from severe stenosis as well as a poor 3-D depiction of the anatomy for surgical planning.